The porous board is probably asfaboard. Acts as a wind barrier. But it and exterior paneling function best with an air gap. If the board gets damp (like here) it does not dry and instead dampens the panel from the inside quite thoroughly!
Today we had a carpenter over who will help us remove everything that is rotten and put in new fresh material. We have removed a bit more of the panel and the board behind it. It is just over 1 meter that is bad and needs to be replaced.
We will take down all the paneling and the board behind it, then put up new ones in the right way (air gap).
Not exactly what the money was supposed to be used for this spring, but that's how it is to own an older house. Hopefully, we'll get rid of the musty smell in the room inside as we feel that the board smells really musty now that we've opened it up.
Then you're in luck. it doesn't look like the entire wall has gone its way, seems to be mostly in the corner and not a bigger cost then, as it could have been.
The carpenter was on site for a few hours and removed the damaged parts and put in new ones. They had to remove about 1 meter, and it was probably the old decking that was the problem.
Exterior drywall and mounting battens, followed by new siding. We also took the opportunity to replace both the old front door and the window. Now we only need some sheet metal and another coat of paint, and then we're done. We'll see if we can manage the planned decking outside or if we'll have to live with the makeshift pallet stairs for a while.
